In the normal practice of aromatic extraction, solvent sulfolane inventory is recycled at a very high rate and during each cycle it is heated and cooled. As a result it undergoes degradation producing acidic materials. It is desirable to remove these acidic materials since, if they are allowed to accumulate, the presence of the acids can result in the corrosion and erosion of equipment. Furthermore, when the solvent accumulates large levels of impurities, its boiling point increases and it generally becomes unusable. As a result, such contaminated or spent sulfolane is generally drained off the system and discarded. One way or another, therefore, removal of these impurities or in other words regeneration of spent sulfolane would be advantageous.
Since degradation of sulfolane generates acidic materials, it is conventional to measure the level of contamination of the solvent in terms of acid value and pH. Acid value of the sulfolane is defined in terms of milligrams of alcoholic potassium hydroxide required to neutralize a one gram sample of solvent and is determined titrametrically using a potentiometer or an acid-base visual indicator suitable for non-aqueous titration. The pH of the sulfolane is measured after diluting it with an equal volume of distilled water. It may be noted that the pH of the sample is an indication of the presence of only strong and moderately weak acidic materials, whereas acid value gives the total acidity. Development of color reduces the transmittance of the sulfolane, and therefore, it is determined spectro-photometrically using distilled water or fresh sulfolane, which is colorless, as a reference.
It is known to employ monoethanol amine (MEA) for the neutralization of the acidic products of sulfolane degradation but the MEA salts formed are not completely stable and can decompose at the temperature of operation. Neutralization with sodium hydroxide forms stable salts but it has an adverse effect on solvent stability. Hence, if the solvent is not to be discarded altogether, it becomes essential to regenerate spent sulfolane.
According to the known art, regeneration of spent sulfolane can be done by one of the following methods, viz. vacuum or steam distillation, back extraction and adsorption. Regeneration by vacuum distillation only removes high boiling materials and involves relatively high capital cost. Regeneration by steam distillation is relatively less capital intensive but also has the limitation of removing only high boiling materials. Regeneration by back-extraction, on the other hand, is a more rigorous method and removes both light and heavy degradation products. However, it is the most expensive regeneration process. Regeneration by adsorption on activated alkaline alumina is satisfactory in the case of highly contaminated solvent but when the acid content is low, as it is in normal operation, the regenerated solvent becomes alkaline and less stable than before.